Some of the scripting languages [Ex: Shell, Bash, python, Ruby, Nodejs, etc.]
Windows, Linux OS commands and operations.
They can learn incrementally also depends on the project need. Note all projects will not use the unique tools. Depends on the IT organization plans, practices and the environments they decide on choosing the vendor based or open source tools.
Note: Some of the famous tools only it has been mentioned. Hence one need to identify the customer project environment and their DevOps architecture also. If one understands the Basic process in their 1st learning phase, later on they can pickup faster.
If you want to learn DevOps Practices, join the below group: